By combining observations and numerical simulations, we investigated the responses of the surface energy budget and the convective boundary layer (CBL) dynamics to the presence of aerosols. A detailed data set containing (thermo)dynamic observations at CESAR (Cabauw Experimental Site for Atmospheric Research) and aerosol information from the European Integrated Project on Aerosol, Cloud, Climate, and Air Quality Interactions was employed to design numerical experiments reproducing two typical clear-sky days, each characterized by contrasting thermodynamic initial profiles: (i) residual layer above a strong surface inversion and (ii) well-mixed CBL connected to the free troposphere by a capping inversion, without the residual layer in between. A large-eddy simulation (LES) model and a mixed-layer (MXL) model, coupled to a broadband radiative transfer code and a land surface model, were used to study the impacts of aerosols on shortwave radiation. Both the LES model and the MXL model results reproduced satisfactorily the observations for both days. A sensitivity analysis on a wide range of aerosol properties was conducted. Our results showed that higher loads of aerosols decreased irradiance imposing an energy restriction at the surface, delaying the morning onset of the CBL and advancing its afternoon collapse. Moderately to strongly absorbing aerosols increased the heating rate contributing positively to increase the afternoon CBL height and potential temperature and to decrease Bowen ratio. In contrast, scattering aerosols were associated with smaller heating rates and cooler and shallower CBLs. Our findings advocate the need for accounting for the aerosol influence in analyzing surface and CBL dynamics.

Abstract. Interpretation of observed diurnal carbon dioxide (CO 2 ) mixing ratios near the surface requires knowledge of the local dynamics of the planetary boundary layer. In this paper, we study the relationship between the boundary layer dynamics and the CO 2 budget in convective conditions through a newly derived set of analytical equations. From these equations, we are able to quantify how uncertainties in boundary layer dynamical variables or in the morning CO 2 distribution in the mixed-layer or in the free atmosphere (FA) influence the bulk CO 2 mixing ratio.We find that the largest uncertainty incurred on the midday CO 2 mixing ratio comes from the prescribed early morning CO 2 mixing ratios in the stable boundary layer, and in the free atmosphere. Errors in these values influence CO 2 mixing ratios inversely proportional to the boundary layer depth (h), just like uncertainties in the assumed initial boundary layer depth and surface CO 2 flux. The influence of uncertainties in the boundary layer depth itself is one order of magnitude smaller. If we "invert" the problem and calculate CO 2 surface exchange from observed or simulated CO 2 mixing ratios, the sensitivities to errors in boundary layer dynamics also invert: they become linearly proportional to the boundary layer depth.We demonstrate these relations for a typical well characterized situation at the Cabauw site in The Netherlands, and conclude that knowledge of the temperature and carbon dioxide profiles of the atmosphere in the early morning are of vital importance to correctly interpret observed CO 2 mixing ratios during midday.

Investigating the influence of surface friction on the inertial oscillation (IO) of an extratropical, non-growing, convective boundary layer (CBL), we paid particular attention to the stability-dependent interactive coupling of shear-induced turbulence and turbulent friction, which leads to a nonlinear relationship between friction and velocity. We showed that in contrast to common perception, IO damping is controlled not only by friction but also by the dependence of friction on velocity. Furthermore, we found that surface friction not only causes damping but also modifies the restoring force. Using these basic principles, we studied the oscillatory properties (equilibrium, periodicity and damping) of the CBL by means of a model based on Monin-Obukhov surface-layer similarity (MOS) and the mixed-layer approximation. We found that the model complies with a quadratic surface stress-velocity relationship (QS) in the neutral limit, and a linear surface stress-velocity relationship (LS) in the proximity of the free-convective limit. Dynamically, the LS leads to a harmonic oscillation with a constant periodicity and exponential damping of the IO. However, the QS displays rather complex anharmonic behaviour; in comparison with the LS it produces a 50% stronger overall damping and a 100% larger contribution to the restoring force. Considering CBLs of arbitrary stability, we found that the MOS stress-velocity relation can be very well approximated by a much simpler linear combination of the LS and the QS which, respectively, represent the convective and the shear-induced contributions to friction. This enabled us to link the set of the external parameters (surface roughness, surface buoyancy flux and boundary layer depth) to a set of three effective parameters: the equilibrium velocity, the convective friction constant and the neutral friction constant. Together with the Coriolis coefficient, these parameters completely determine the IO.Key Words: wind oscillation; Monin-Obukhov surface-layer similarity; parametric oscillator; oscillator properties; mixed-layer model; free convection limit

Interpretation of observed diurnal carbon dioxide (CO₂) mixing ratios near the surface requires knowledge of the local dynamics of the planetary boundary layer. In this paper, we quantify the relationship between the boundary layer dynamics and the CO₂ budget in convective conditions through a newly derived set of analytical equations. From these equations, we are able to quantify how uncertainties in boundary layer dynamical variables or in the morning CO₂ distribution in the mixed-layer or in the free atmosphere influence the bulk CO₂ mixing ratio. <br><br> We find that the largest uncertainty incurred on the mid-day CO₂ mixing ratio comes from the prescribed early morning CO₂ mixing ratios in the stable boundary layer, and in the free atmosphere. Errors in these values influence CO₂ mixing ratios inversely proportional to the boundary layer depth (<i>h</i>), just like uncertainties in the assumed initial boundary layer depth and surface CO₂ flux. The influence of uncertainties in the boundary layer depth itself are one order of magnitude smaller. If we "invert" the problem and calculate CO₂ surface exchange from observed or simulated CO₂ mixing ratios, the sensitivities to errors in boundary layer dynamics also invert: they become linearly proportional to the boundary layer depth. <br><br> We demonstrate these relations for a typical well characterized situation at the Cabauw tower in the Netherlands, and conclude that knowledge of the temperature and carbon dioxide vertical profiles in the early morning are of vital importance to correctly interpret observed CO₂ mixing ratios during midday

The construction and deployment of a portable trace-gas measurement system (TGaMS) is described. The air-collection system (dubbed HYDRA) collects air samples from 18 different locations and was connected to either one or two LI-COR LI-7000 gas analyzers to measure CO 2 . An in situ ''field calibration'' method, that uses four calibration gases with an uncertainty on the order of 60.1 mmol mol 21 relative to the WMO CO 2 mole fraction scale, revealed CO 2 output from the LI-7000 had a slightly nonlinear relationship relative to the CO 2 concentration of the calibration gases. The sensitivity of the field-calibrated CO 2 to different forms of the field-calibration equation is investigated. To evaluate TGaMS performance, CO 2 from collocated inlets, portable gas cylinders, and nearby independent CO 2 instruments are compared. Results are as follows: 1) CO 2 measurements from HYDRA multiple inlets are feasible with a reproducibility of 60.4 mmol mol 21 (based on the standard deviation of the CO 2 difference between collocated inlets when HYDRA was operating with two LI-7000s); 2) CO 2 differences among the various field-calibration equations were on the order of 60.3 mmol mol 21 ; and 3) comparison of midday hourly CO 2 measurements at 30 m AGL between TGaMS and an independent high-accuracy CO 2 measurement system (within 300 m of TGaMS) had a median difference and standard deviation of 0.04 6 0.81 mmol mol 21 over two months.